1. Field Of The Invention
The invention relates to a method of breaking an aqueous emulsion, in particular an oil-in-water emulsion, as well as to a device for carrying out this method.
2. Background Art
Oily emulsions are generated in industry by numerous processes, including drilling and cutting emulsions in mechanical metal working, which are used for cooling and lubrication. These are predominantly oil-in-water emulsions on a mineral oil basis which have been stabilized by emulsifiers and optimized for the respective applications by further additives. In addition, oily waste water is generated by the cleaning of oily and fatty surfaces. Large amounts of oily water result from the production, transport and processing of petroleum.
Breaking of the emulsions may be desirable for two different reasons. On the one hand, it is one of the most important tasks of waste water treatment in order to exclude pollution of surface and ground waters and, on the other hand, breaking is desirable in order to remove the oil content--in particular in petroleum production.
A plurality of methods is known for breaking emulsions. In addition to the classic methods such as distillation, breaking by means of acids, and adsorption, ultrafiltration units have recently been developed. However, these methods involve the disadvantage that the filtration rate decreases with increasing oil concentration, and that ultrafiltration membranes are sensitive to extreme pH values and pollution. Additional disadvantages are that complex processes, chemical additives, and secondary wastes are involved.
In addition, methods have been proposed where emulsions are broken by means of electric current. The method of electroflotation involves the separation of the oil droplets from broken emulsions by means of gas bubbles which can be generated electrolytically in the waste water, for example, by means of plane-parallel perforated electrodes provided on the bottom of the container.
It is necessary, however, first to break the emulsions chemically, e.g., by adding salts or acids and possibly flocculants. This results in a substantial increase in salt concentration in the waste water [E. H. Baer, VDI-Berichte 185, (1972)].
According to another electromechanical method, ferrous salts are introduced into the suspension by anodic dissolution of iron or steel anodes; these ferrous salts are particularly voluminous hydroxides which chemically break the emulsion. In order to reach sufficient conductivity in this case and to prevent passivation of the electrodes, it is necessary to add salts. In this case, also, the salt concentration rises, and the oil is obtained as a sludge of high water content [H. H. Weintraub et al., Electrochem. Soc. Spring Meeting, (May 1976) Abstract No. 261].
In addition, a method of electrophoretically separating emulsions in an electrochemical cell has been proposed. The cell is divided by a diaphragm into an anode chamber and a cathode chamber. The emulsion is fed into the cathode chamber, a d.c. voltage being applied. The negatively charged oil droplets migrate through the diaphragm under the influence of the electric field into the anode chamber where the emulsion is broken by electrochemical action. The oil is obtained as a separate oil layer [D. D. Snyder and R. A. Willihnganz, Proc. Ind. Waste Conf. 31 (1977), 782-791]. This method operates without the addition of chemicals and prevents increases in the salt concentration in the water. A disadvantage of this method is that the breaking efficiency is determined by the migration rate of the oil droplets in the electric field and through the membrane, and, thus, is largely dependent on the type of emulsion. In particular, non-ionic emulsions cannot be broken by this method--electrophoretic migration of the emulsified oil droplets. This means a very strong restriction of its field of application. In addition, high demands are being made on the diaphragm: on the one hand, its porosity must be so high that the migration is not seriously impeded and, on the other hand, the pores may not be so large that they permit marked back-diffusion. The properties are not to change in the course of a long operation time--for example, as a result of pollution.
German Published Patent Application No. 2910314 describes a method of separating particles from suspensions, an electric charge opposite to that of the suspended particles being supplied to the suspension, and the coagulating particles being separated by means of known methods. In order to achieve a sufficiently high electric conductivity, the suspension has to be made strongly acid prior to the separation process.